The apparatus of this invention can be used for the powder filling of molds, for example, the molds used in hot isostatic pressing, dry bag cold isostatic pressing, and wet bag cold isostatic pressing. Cold isostatic pressing is described, for example in "Isostatic Pressing Technology", edited by P. J. James, Applied Science Publishers Ltd., New York, 1983, pp. 91-161, incorporated herein by reference. Cold isostatic pressing is the compaction of powders by pressing at low or ambient temperatures. A compact is formed having a green strength due to the mechanical interlocking of the powder particles. Some of the products made by cold isostatic pressing include spark plug insulators, china, and solid fuel rods. The pressed or green compact can be sintered by high temperature heating to achieve higher density and improved mechanical strength. Two of the well known cold isostatic pressing methods and the tooling associated therewith are known as the wet bag cold isostatic press and the dry bag cold isostatic press.
In dry bag cold isostatic pressing, sometimes herein referred to as dry bag pressing, an elastomeric bag or mold is fixed within a pressure vessel. The elastomeric mold has at least one open end which is sealed with the pressure vessel so that the fluid pressure medium within the vessel cannot enter the mold interior. The elastomeric mold is made from a material which does not chemically react with either the powder or the pressure medium, and readily releases from the green compact after pressing. For example, a cylindrical elastomeric mold having a high length to diameter ratio is open at both ends, and has a cylindrical void space therein. Sealing means for the open mold ends are provided by wear resistant metal punches. The punches are located and restrained by the yolk of the press, and guided into the bag by wear resistant bushes mounted in the pressure vessel. The top punch is removed and powder is charged into the void space in the mold by completely filling the void space between the sealing means. The powder filling apparatus of this invention is particularly useful in filling such molds used in dry bag or wet bag cold isostatic pressing.
An important step prior to cold isostatic pressing is powder filling of the mold cavity, preferably to a uniform fill density from top to bottom. As used herein, the term "fill density" means the density of powder in the mold prior to compaction. The uniformity of powder density in the filled mold translates into dimensional uniformity of the pressed compact. When the powder does not have a uniform fill density various non-uniformities and defects can be found in the pressed compacts. In some methods, powder is spray dried with a binder before filling to create agglomerates that will improve flowability. However, such binders can be difficult to remove and remain as undesirable contaminants in the pressed compact.
In the case of tungsten powder or other fine powder that does not freely flow, and where a binder is undesirable, cold isostatic pressing molds having a high length to diameter ratio are difficult to fill to a uniform powder density. For example, the free fall of a high density powder, such as tungsten, to the bottom of the mold can result in a higher packing at the bottom of the mold, whereas the top of the mold has a loose packing. After cold isostatic pressing, the variation in fill density produces compacted parts with a slightly conic profile where the bottom has a larger diameter. Uniform powder filling also minimizes necking in the pressed compact, usually caused by underfill or powder settlement prior to cold isostatic pressing.
An important parameter in the design and operation of the isostatic press and mold is the "compaction ratio" of the powder. The compaction ratio is the ratio between the initial fill density of the powder in the mold, and the density of the compact after isostatic pressing. The rigid punches sealing the elastomeric mold do not deform during isostatic pressing, and as a result a flare forms in the compact at the interface where the powder meets the sealing punches. The flare, sometimes known as elephant's foot, is proportional to the compaction ratio of the powder. A high powder fill density lowers the compaction ratio, and therefore reduces such flaring of the compact ends adjacent sealing punches in the mold.
Avoidance of non-uniformities and defects in pressed compacts is of greater importance in compacts subjected to additional processing, such as tungsten compacts that are sintered and wire drawn to form the filament wire in incandescent light bulbs, or molybdenum compacts that are sintered and wire drawn to form wire leads and supports in incandescent light bulbs. For example, such non-uniformities and defects in pressed compacts as described above can cause failure during wire drawing, or premature failure of the drawn filament in use as an incandescent light.
Another problem to be avoided in powder filling cold isostatic pressing molds, especially with finer powder of 2 microns or less, is the entrapment of excessive amounts of air which can be trapped between powder particles during filling. During compaction of the powder in the cold isostatic press, the entrapped air remains distributed throughout the compact in the form of small voids or high pressure pockets of air. After the cold isostatic pressing is completed the mold and compact must be decompressed. During decompression of the mold, the entrapped air pockets can apply non-uniform forces to the compact and result in breakage or damage to the compact.
It is an object of this invention to provide an apparatus for powder filling molds to a uniform fill density.
It is another object of this invention to provide an apparatus for powder filling generally cylindrical molds to have a high and uniform fill density that minimizes nonuniformities and defects in pressed compacts.